This invention generally relates to hot-melt bookbinding, books and related articles bound thereby, a method for applying such adhesives and a system for performing the method. More specifically, this invention relates to two-shot hot-melt bookbinding developments which utilize a low viscosity polymer-based primer adhesive and a high viscosity polymer based cover glue. The primer adhesive of the present invention is suitable for use in the binding of books made with difficult to bond paper substrates. Books manufactured with the disclosed two-shot hot-melt formulations exhibit desirable "easy open" properties while avoiding the problems of "cold cracks" and "cold flow".
As known in the art, perfect binding was originally used as a fast and inexpensive way to bind paperbacks or pocket books. Today, however, perfect binding is used to bind a wide variety of articles such as magazines, catalogs, directories, and the like. As used herein, "books" shall refer to all such bound articles. The technique of perfect binding generally requires stacking of the sheets of paper which will make up the pages of the book. The stacked pages are referred to as a "signature". Generally, a number of signatures are stacked together to form a "book block" which is held within a clamp while adhesive is applied to a prepared edge or backbone thereof. A cover sheet or strip is then contacted with the edge or backbone before the adhesive is allowed to set.
Prior two-shot adhesive methods often have employed primer coats formulated with an emulsion adhesive which will generally penetrate the backbone even when difficult paper substrates such as glossy paper, for example, are used. In such prior art two-shot systems, the primer coat is typically a low viscosity water-based vinyl acetate ethylene (VAE) emulsion with an ethylene vinyl acetate (EVA) or a styrene butadiene styrene rubber (SBS) hot-melt cover glue. When applied, the primer penetrates the backbone via capillary action and is dried in a conventional manner. The cover glue, in the form of a high temperature melt, is then applied to the primer and to the liner or soft paper cover. Books and other articles which have been bound by this method often experience delamination between the primer coat and the cover glue due to improper dying of the primer, resulting in poor adhesion of the cover glue thereto.
Other two-shot systems have included compatible hot-melt adhesives, such as EVA based adhesives, as both the primer and the cover glue. The use of such systems has been found useful in combatting the problem of delamination between the primer and the cover glue since the two hot-melt formulations are chemically compatible to thereby promote mixing between the two hot-melt coatings. With regard to EVA based hot-melt adhesives, the hot-melt primer typically has a viscosity of 3,000 to 4,000 cps at 350.degree. F., and the cover glue typically has a viscosity significantly greater than that of the primer. Such two-shot hot-melt formulations generally promote stronger and higher quality bookbinding when compared with books which have been bound using a one-shot method, i.e. bound with a single hot-melt adhesive. However, traditional hot-melt two-shot methods have been less than satisfactory especially when used to bind difficult paper substrates. In such applications, the viscous hot-melt primer is not readily absorbed into the backbone of the book block. Therefore, two-shot polymer based adhesives have not been particularly useful in making strong books having the same easy open qualities of books bound by two-shot systems which include emulsion primers.
The present invention overcomes the above-discussed problems of the prior art by providing a two-shot hot-melt system wherein both the primer and cover glue formulations are hot-melt adhesives, wherein the primer is a low viscosity hot-melt adhesive and wherein the cover glue is a high viscosity hot-melt adhesive. Preferably, both the primer and cover glue are EVA hot-melt formulations which are chemically compatible to thereby avoid the aforementioned delamination problem. Additionally, the low viscosity hot-melt primer readily penetrates within and adheres to the backbone of the book block. In this manner, high quality paperback and hard bound books are easily and routinely manufactured even with paper substrates that present the most difficult adherence problems.
The present invention provides a two-shot hot-melt bookbinding system, books and related articles bound thereby, a method for perfect binding with the disclosed adhesives and a system for practicing the disclosed method. The formulations of the present invention include a low viscosity primer preferably having a viscosity between about 1,100 and about 2,500 cps and including between about 35 and about 45 percent by weight of EVA polymers. Other known ingredients, such as high melting point waxes, tackifiers and stabilizers are also typically included in the disclosed primer formulations. Additionally, high viscosity cover glue formulations are disclosed having a preferred viscosity between about 4,000 and about 6,000 cps and between about 40 and about 50 percent by weight of EVA polymers. The disclosed cover glue formulations may also include various additional known ingredients such as high melting point waxes, tackifiers, stabilizers, and pigments.
As discussed further below, books and related articles which are bound by the disclosed formulations can be prepared without the use of the gas heaters which are generally required to keep the hot-melt primer in molten form prior to application of the cover glue. Hence, the proper glue pot temperature and a longer hot-melt open-time allows the primer to remain molten after its application to the book block and prior to the application of the cover glue.
Accordingly, it is an object of the present invention to provide two-shot hot-melt adhesive formulations.
It is another object of the present invention to provide two-shot hot-melt adhesive formulations, including primer and cover glue formulations, which contain EVA polymers.
It is still another object of the present invention to provide two-shot hot-melt formulations based on EVA polymers and having a low viscosity primer which will penetrate a paper substrate and a high viscosity cover glue which will penetrate, fuse, and re-enforce the primer.
It is still another object of the present invention to provide two-shot hot-melt formulations based on EVA polymers and articles which are bound with such two-shot hot-melt formulations.
It is still another object of the present invention to provide a two-shot perfect binding method which utilizes the hot-melt adhesive formulations disclosed herein.
It is still another object of the present invention to provide a system for practicing a method for two-shot hot-melt bookbinding and utilizing the adhesive formulations disclosed herein.